Briefly, this error occurs when Elasticsearch encounters an issue while trying to replicate data for shard action. This could be due to network issues, insufficient disk space, or a problem with the underlying hardware. To resolve this, you can check the network connectivity between the nodes, ensure there is enough disk space, and verify the health of the hardware. Additionally, check the Elasticsearch logs for more detailed error information. If the issue persists, consider reindexing the data or increasing the number of replicas.
This guide will help you check for common problems that cause the log ” unexpected error while replicating for action [{}]. shard [{}]. ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: replication.
Overview
Replication refers to storing a redundant copy of the data. Starting from version 7.x, Elasticsearch creates one primary shard with a replication factor set to 1. Replicas never get assigned to the same node on which primary shards are assigned, which means you should have at least two nodes in the cluster to assign the replicas. If a primary shard goes down, the replica automatically acts as a primary shard.
What it is used for
Replicas are used to provide high availability and failover. A higher number of replicas is also helpful for faster searches.
Examples
Update replica count
PUT /api-logs/_settings?pretty
{
"index" : {
"number_of_replicas" : 2
}
}Common problems
- By default, new replicas are not assigned to nodes with more than 85% disk usage. Instead, Elasticsearch throws a warning.
- Creating too many replicas may cause a problem if there are not enough resources available in the cluster.
Log Context
Log “unexpected error while replicating for action [{}]. shard [{}].” classname is TransportReplicationAction.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :
return pending.get();
}
Override
public void onFailure(Throwable t) {
logger.error("unexpected error while replicating for action [{}]. shard [{}]. "; t; actionName; shardId);
forceFinishAsFailed(t);
}
/**
* start sending replica requests to target nodes
